Phosphor coating suspension



2,841,562 PHOSPHOR coarnse SUSPENSION No Drawing. Application November 5, 1954 Serial No. 467,220

4 Claims. (Cl. 252-3013 This invention relates to coating fluorescent lamps, tubes and similar devices with phosphors suspended in an organic binder which is afterward removed by baking. The organic binder includes a plastic material, such as a cellulose, a solvent therefor, and generally also a plasticizer and a dispersing agent.

These ingredients are difficult to remove completely, and yet even a very small percentage remaining in the coating is deleterious to the light output of the lamp. The residue remaining in the coating is generally in the form of carbon, or a carbon containing compound, due to decomposition of the binder at the baking temperature. I have found that a flow of substantially pure oxygen over the coating during the latter portion of the baking process will remove the last'trace' of the binder, and will improve the luminescent output from the coating, despite the belief heretofore that the oxidation of the phosphor which would occur at the elevated temperature required in the baking process would be deleterious to the eificiency, especially when a manganese activator is present. I i The use of'an oxygen ambient atmosphere during baking is expensive and difficult to handle under conditions of mass production. I have further discovered, however, that the addition of an oxidizing agent to the coating before baking, and preferably to the coating suspension before. application to the device, will remove the residual binder in a much less expensive and more convenient manner, suitable for use in mass production. Nitrated organic oxidizing agents such as, for example, amyl nitrate and nitrocarbamates of such alcohols as isopropyl, ethyl, methyl and the like were found to be especially useful. The nitrocarbarnate has the additional advantages of acting as a dispersing agent and also as a plasticizer, thereby reducing the amount of plasticizer needed in the coating suspension. The multiple action of the nitrocarbamate is particularly desirable, because ordinary plasticizing agents are the most difiicult part of the binder to remove.

Because of the efiectiveness of nitrocarbamates, amyl nitrate and the like, as dispersing agents, a considerable reduction in the weight of powder necessary for a given coating density is achieved. The latter results in a considerable saving in the cost of manufacture of the lamp, because phosphor is expensive.

The results of adding an oxidizing agent, isopropyl nitrocarbamate, to a standard coating suspension in place of the dispersing agent is shown below.

-. ates Patent Table II Powder Initial Amount amyl nitrate by weight Weight lumens per watt output None (control) 5. 39 62. 0 5% 4. 77 62. 6

The table-is for a coating density of 79.1, and the powder weight is seen to be considerably reduced by the use of the amyl nitrate.

In addition to the gain secured by reduction in powder weight for a given density of phosphor coating, the use of an oxidizingagent according to the invention secures an even greater gain by permitting the use of a lower density than would otherwise be satisfactory, as shown in the tests summarized in the table below.

Table III Lumens per watt at operating hours L. P. W.

Density Powder indicated loss from Weight -1,750

. hours Contr0I 78. 8 4. 70 62. 4 58. 6 53. 4 6. 2 Special I. 78. 7 4. 35 63.0 60. 6 58.0 2. 6 Special II 77. 4 3. 60 61.0 60. 2 57. 6 2. 6

In the above tests, the control was a standard coating composition of a type heretofore known and containing no oxidizing agent. The tests marked Special I and Special 11 were made to ditfering densities with a coating suspension containing an oxidizing agent, one density being substantially the same as the control and the other lower.

In addition to the decrease in powder weight and improvement in brightness-maintenance resulting from the addition of an oxidizing agent, I find also that an increase in initial brightness can be achieved by the use of binders which are more easily removable by oxidizing agents. For example, by using a T-type ethyl cellulose as a binder instead of the usual N-type, a considerable gain is obtained. The N-type contains about 48% ethoxyl whereas the T-type contains 49% or more.

The, table below shows the gain in initial brightness obtained by the use of the new coating suspension, containing both isopropyl nitrocarbamate and T-type ethyl cellulose.

Table IV Lumens per watt at operating hours Density P. W. indicated Control .Q 79. 9 5. 71 e1. 0 60. 0 Special 78. 6 4. 30 62. 8 62. 2

In each table above, the control lacquer was made by mixing 1 part by weight of 200-centipoise N-type ethyl cellulose (Hercules N-200) with 1.5 parts of dibutyl phthalate and enough solvent to give a viscosity of 15 seconds. The solvent was 95% xylol and 5% butanol. About 250 cc. of the above 15 second lacquer was mixed with 300 grams of calcium halophosphate phosphor, activated by manganese and antimony, of the type described in United States Patent 2,544,663 to D. F. Fortney and G. Moran, and having a particle size of about 6 microns. About 1.5 grams of Alon C, a superfine aluminum powder described in a copending patent application Serial No. 382,480, filed September 25, 1953, by Taubner and Homer, was added to the suspension, and about 1.14% of Armeen CD by weight of phosphor, a dispersing agent containing by weight 8% octyl, 9% decyl, 47% dodecyl, 18% tetradecyl, 8% hexadecyl, 5% octadecyl, and 5% octadecanol primary amines.

In Table I, the given percentages of the isopropyl nitrocarbamate oxidizing agent were added to the above suspension in place of the dispersing agent. That is, the Armeen CD was omittedfrom all suspensions except the control, and the isopropyl nitrocarbamate added. In Table II, the addition was amyl nitrate instead of the nitrocarbamate.

In the other tables the controls were the same but the Special suspensions were made by dissolving one part of isopropyl nitrocarbamate, one part of 20 -centipoise T-type ethyl cellulose (Hercules T-200) and 012 part dimethyl phthalate, in a solvent of 95 xylol and 15% butanol, the amount of the solvent being just enough to give a viscosity of about 15 seconds. 300 gramsof the phosphor powder above and 1.5 grams of Alon C were suspended in 250 cc. of the 15 second lacquersolution described.

All suspensions, after milling, are generally adjusted by adding a small additional amount of the 15 se'c'ond lacquer solution until the coating density has the desired value.

In order to reduce the amount of unoxidized material, or carbon, in the coating, I prefer to use the ethyl cellulose lacquer previously described. The addition to such a lacquer of the oxidizing agent is especially effective, although such agents also Work effectively with other lacquers, as noted above. A

The special ethyl cellulose lacquer not only uses T-200 cellulose instead of N200, thereby improving the baking out properties, but also uses dimethyl phthalate instead of dibutyl phthalate, thus still further reducing the baking out properties, and in addition uses isopropyl, methyl, ethyl or other nitrocarbamate-s, each of which notonly acts as oxidizing agent but also as a plasticizer and dispersing agent as well, thereby reducing the amount of additional plasticizer such as dimethyl phthalate needed and eliminating entirely the need for an additional dispersing agent.

After the lamp tube is coated with the phosphor suspension, the coating is dried and baked in the manner customary in the art, a baking temperatureof about 400 C. being preferred. The time of baking can be about=l minutes. The tube is then made into a fluorescent lamp in the manner customary in the art.

In the tests reported above, the tube coated was the usual tube for a 40-watt fluorescent lamp, that is it was a tube 48 inches long and 1 /2 inches in diameter. The lumens per Watt data was taken after the tube was made into a 40-Watt lamp of a type standard except for the coating which was changed as specified.

The density of the coating given in the tables above is a relative figure and is obtained by measuring the percentage of ,light transmitted through the coating from the beam of a standard incandescent lamp.

The designation 200-centipoise for the ethylcellulose means that the ethyl cellulose when in a solutionof 5 grams of nitrocellulose in 80 cc. of ethanol and 20 cc.

1- of toluol has a viscosity of 200 centipoises. That is the standard solution for rating ethyl cellulose for viscosity.

Although a calcium halophosphate phosphor has been used in the data reported above, that is merely by way of example and not by way of limitation. The invention can also be used with other phosphors such as calcium tungstate, magnesium tungstate, and zinc orthosilicate, magnesium germanate and others. The tungstates can be used either self-activated, or with a lead activator; the orthosilicates can have a manganese activator, and so can the germanates.

In certain high pressure mercury lamps having a sealed quartz mercury arc tube enclosed in a larger glass envelope, a coating of magnesium germanate phosphor is generally applied to the inner surface of the outer envelope to increase the output of red light. The phosphor is generally applied in a nitrocellulose lacquer, which is afterward baked out in the manner customary for fluorescent lamps. A very high reduction 'in brightness is generally observed, the brightness dropping from its initial value of 18,000 lumens to a value of 16,000 lumens in hours. The addition of a few percent of an oxidizing agent such as those mentioned herein eliminates this initial drop in brightness.

The particular phosphors mentioned above are only examples; others can be used.

The oxidizing agent will not be as effective as shown above if the amount used is less than about 1% 'by weight.

A method of making nitrocarbamates is given in Carbamates and N-Nitrocarbamates by Howard M. Curry and I. Philip Mason in the Journal of the American Chemical Society, volume 73, page 5043, of the year 1951.

,What I claim is:

1. A phosphor coating suspension comprising a powdered phosphor, an organic binder therefor, and mixed therewith a smallpercentage, at least 1% by weight of a nitrocarbamate of a radical selected from the group consisting of: isopropyl, methyl, ethyl.

2. A phosphor coating suspension comprising: an ethyl cellulose binder having a high ethoxyl content; dimethyl phthalate; sufiicient solvent to dissolve the ethyl cellulose and form a suspension having a viscosity of about 15 seconds; a powdered phosphor; and between about 1% and about 10% of an oxidizing agent selected from the group consisting of isopropyl nitrocarbamate, methyl nitrocarbamate and ethyl nitrocarbamate.

3. The suspension of claim 2, in which the ethoxyl content of the ethyl cellulose is at least 49% by weight of the ethyl cellulose.

4. A phosphor coating suspension comprising: an ethyl cellulose binder having a high ethoxyl content; dimethyl phthalate; sufiicient solvent to dissolve the ethyl cellulose and form a suspension having a viscosity of about 15 seconds; a powdered phosphor; and between about 1% and about 10% of an oxidizing agent selected from the group consisting of isopropyl nitrocarbamate, methyl nitrocarbamate and ethyl nitrocarbamate; said suspension being free from any additional plasticizing lor dispersing agents.

References Cited in the file of this patent UNITED STATES PATENTS 2,172,046 Cortese Sept. 5, 1939 2,181,305 Myers Nov. 28, 1939 2,524,733 Payne Oct. 3, 1950 2,678,888 Evans May 18,1954 2,689,830 Sadowsky Sept. 21, 1954 OTHER REFERENCES 7 Gregory: Uses and Applications of Chemicals and Related Materials, (1939), page 484. 

1. A PHOSPHOR COATING SUSPENSION COMPRISING A POWDERED PHOSPHOR, AN ORGANIC BINDER THEREFOR, AND MIXED THEREWITH A SMALL PERCENTAGE, AT LEAST 1% BY WEIGHT OF A NITROCARBAMATE OF A RADICAL SELECTED FROM THE GROUP CONSISTING OF: ISOPROPYL, METHYL, ETHYL. 